RXD, OCD are outputs and TXD, RTS are inputs. These four connections have to be level shifted for RS 232 to 3.3V TTL communication. Therefore, you can use the MAX3232 to accomplish this task. Your computer running the HART stack will then be able to use these 4 pins to successfully transmit over the current loop. Figure 4 in the DS8500-KIT datasheet shows a simplified connection diagram for Master evaluation.